Ceramic matrix composites (“CMCs”) have high temperature capability and are light weight. The composites are thus an attractive material for various applications, such as for components in gas turbine engines where temperature durability and weight are important considerations. Current methods of preparing CMC products involve forming a laminate of ceramic fiber and matrix, thermally treating the laminate, applying an infiltrant to the laminate, and densifying the laminate. The densified laminate may then be machined to prepare a CMC product with the desired dimensions. Alternatively, woven preforms can be used instead of laminates.
The infiltrant may or may not react with one or more constituents in the preform. For example, during infiltration of molten silicon into a carbon containing preform, the silicon and carbon can react to form silicon carbide. In this case, the volume of silicon carbide formed from this reaction is greater than the volume of carbon that was consumed. The result is that the pore structure that transports silicon through the preform is reduced by this reaction. In the extreme case, the pores can close completely and choke off infiltration. In the case of chemical vapor infiltration (“CVI”), the reaction product of the infiltrating gases deposits on the surface of the pores, thereby reducing the amount of porosity. To successfully infiltrate a preform, the infiltrating fluid should have a percolated path to the infiltration front. This is balanced by the desire to have a fully dense product with a controlled amount of unreacted infiltrant or residual porosity. In melt infiltrated (“MI”) CMCs, judicious selection of reactant particle size, reactant volume fraction, volume fraction of non-reactive preform constituents, pore volume fraction, and reaction stoichiometry is the typical route for maximizing part densification and controlling the final part composition including the amount of unreacted infiltrant. In CVI CMCs, judicious control of pore size and distribution is the typical route for maximizing densification and minimizing residual porosity.
Infiltration of thick preforms is especially challenging when infiltration pathways, such as pores, seal up or choke-off prematurely lowering the overall permeability. For both CVI and MI CMC materials, successful infiltration of thin, small preforms often does not translate well to thicker, larger preforms. In small parts, the infiltration distance is relatively short, such that a part may be fully infiltrated over a reasonable time scale even if the permeability of the matrix becomes low during infiltration. In larger, thicker parts, the infiltration distance is long and often the infiltrant does not reach the more inner areas of the preform. Permeability from the reacted or deposited matrix material may be too low, such as so low as to arrest infiltration completely, resulting in a defective part.
In some ply based MI CMCs, the primary route for infiltrant transport to the reaction front may be channeled elongate porosity remaining between plies after compaction of laminated tapes. Control of the size, shape, and number of these channels in a way that is reproducible is challenging. Variations in tape cured ply thickness, roughness, and autoclave compaction all contribute to variability in the size, shape, and number of these channels. Further, if these voids are still present in the final composite structure, the product may be considered defective.
Thus, an improved method of preparing ceramic matrix composites, particularly using melt infiltration or chemical vapor infiltration, is desirable in the art.